The unidirectional solidification form of the precision investment casting method has been the subject of significant effort in connection with the manufacture of blades and vanes for the turbine section of gas turbine engines. It has been recognized that certain mechanical properties, such as thermal fatigue life, can be improved in the elongated grained or single crystal structures which can be provided by such a method.
Because high temperature superalloys generally are used in the manufacture of such turbine blades and vanes, a wide variety of directional solidification apparatus has incorporated an approach used in the conventional precision casting of such alloys. Such approach involves first melting a charge in a crucible or pour cup and then pouring the molten charge into a mold, all within a heated, vacuum enclosure. This type of method and apparatus is described in such patents as U.S. Pat. Nos. 3,538,981--Phipps, Jr., 3,770,047--Kirkpatrick et al and 3,847,203--Northwood. As is shown in such typical structure, it is common in the pour casting method to use induction heating coils for the application of heat to the mold area or to the crucible for melting of the charge prior to pouring.
Another form of a directional solidification apparatus and method is described in U.S. Pat. No. 3,897,815--Smashey, the disclosure of which is incorporated herein by reference. The Smashey patent, in the embodiment shown in its drawing, describes an enclosed, self-casting type furnace for use in the directional solidification method. In that form, a plurality of heating elements are arranged in a substantial vertical array to heat the furnace chamber in order to control features of the withdrawal method of directional solidification to which that Smashey patent relates.
The present invention, which relates to apparatus of the enclosed, self-casting directional solidification type, employs a refractory mold which includes an upper charge melting cup connected to a lower article casting portion to allow flow of molten charge into such lower portion of the mold. Such a mold is described in U.S. Pat. No. 4,044,815--Smashey et al, the disclosure of which is incorporated herein by reference.
Although the prior art has described a variety of methods and apparatus for use in the directional solidification casting of metal articles, the casting of relatively large articles, for example of about two pounds or more, either has required the separate charge melting and then pouring of molten metal into a mold or has required inordinately long periods of time for resistance furnace melting. As was mentioned before, using the melt-and-pour technique, the charge commonly is melted by induction heating and then poured into the mold cavity by tilting a melt ladle to allow liquid metal to run into the mold. While basically a simple procedure, this process has several serious deficiencies.
One problem is that the melt and pouring ladle is a major source of ceramic-type contamination of the metal being cast. Such contamination eventually is reflected in the form of ceramic inclusions in the finished casting.
A serious problem relates to the pouring of liquid metal into the top of a mold in an enclosure heated by a resistance wound furnace, which is desirable for close control of the solidification portion of the process. Mechanically, it is difficult to direct the entire liquid metal stream continually through the relatively small opening in the top of the mold. As a result, some molten metal invariably is cast onto the furnace internal structure. This can create furnace electrical shorts caused by spattering onto the resistance heating coils. In addition, such misdirected metal, which adheres to the outer portions of the mold, can cause interference of the mold with other furnace portions when the mold is lowered in the withdrawal process.
Another problem associated with the melt-and-pour technique is the difficulty of control of liquid metal superheat prior to pour. In such technique, superheat temperature is measured through an optical pyrometer directed onto the surface of the liquid metal bath through a viewing port or by immersion thermocouple probes. However, error is introduced into such measurement through the formation of slag on the surface being measured and metal vapor collecting on the viewing port, and by mechanical manipulation and insulation problems associated with the immersion probe. Therefore, it becomes difficult to obtain a meaningful temperature reading of the liquid metal bath.